A lot of people (myself included) don't or back in 1967, didn't like the plastic buckle standard belts, so they opted to splurge the extra $6.35 for the A39 Custom Deluxe Seatbelt upgrade. As a matter of fact, 51,247 customers felt this way!

Even deluxe interior cars came with standard belts. Deluxe belts were a separate option that had to be ordered if you wanted it.

And I'm sure that confused a lot of customers who assumed that the Deluxe Interior (Z87) included the Deluxe Seat Belts and found black belts with plastic buckles when their car showed up. My '69 Z/28 is a pretty highly-optioned car (including Z87) and looks odd with black standard belts.